According to the most common solutions, each of the rotor blades comprises an airfoil portion and a root portion, wherein the root portion is received in a corresponding retaining groove formed on the peripheral surface of the rotor disk, and wherein the groove and the root portion have matching shapes and dimensions. In particular, and still according to the most common solutions, in rotor blades of axial insertion type, the root of each rotor blade is usually of a so-called “fir tree” configuration, the matching shapes of the root portion and the groove enabling the blade to be securely attached to the periphery of the rotor disk, in particular enabling to limit radial displacement of the blade during rotation at very high speed of the rotor disk.
However, while for enabling an easy assembly of the blade, in particular for enabling easy insertion of the root portion into the corresponding retaining groove, the dimensions of the root portion and retaining groove are such that a clearance or backlash is usually left between the root portion and the retaining groove in order to allow radial and lateral small movements of the root portion with respect to the retaining groove and/or the disk, axial movements of the root portion need to be avoided.
To this end, in particular for the purpose of securely locking the blade to the rotor disk, several conventional solutions have been proposed. For instance, solutions are known according to which tab washers are inserted into the retaining groove (between the root portion and the groove). Other solutions are known according to which wedge shaped elements are forced between the root portion and the groove—for instance by means of threaded pins engaging corresponding threaded portions of the wedged shaped elements—wherein, due to the radial thrust action exerted by the wedge shaped elements on both the root portion and the groove, the blade is locked on the rotor disk by friction.
However, the above mentioned conventional solutions for securely locking blades on the rotor disk, have several drawbacks.
A first drawback relates to the special machining needed for the root portion and/or the retaining groove that introduces discontinuities on the groove surface which may turn into stress concentration regions and therefore cause structural weakening. If a thread is present on a rotor groove problems can be even worse because the threads may turn into crack initializations.
Still by way of example, a further drawback relates to the fact that, in many of the conventional solutions threads are present and therefore caulking is needed. However, caulking operations are very often troublesome due to strict assembling requirements. Known friction based or interference based retaining methods, applied to the coupling between the blade root and corresponding groove, depend heavily on assembly and operating conditions and therefore cannot always be employed.